1. Field of the Invention
The present invention relates to a multi-wavelength optical modulation circuit that demultiplexes multi-wavelength light, modulates the light of each wavelength channel using a plurality of optical modulators, performs wavelength division multiplexing on the modulated signal light, and then transmits it, and also relates to a wavelength-division multiplexed optical signal transmitter that uses this multi-wavelength optical modulation circuit. In particular, the present invention relates to a multi-wavelength optical modulation circuit and wavelength-division multiplexed optical signal transmitter that can demultiplex wideband multi-wavelength light output from a multi-wavelength generation light source into individual wavelength channels. In addition, the present invention relates to a multi-wavelength optical modulation circuit and wavelength-division multiplexed optical signal transmitter that can suppress power level deviations between each wavelength channel.
2. Description of the Related Art
In order to meet the demands in recent years for increased transmission capacity, the development of wavelength-division multiplexing (WDM) transmission systems in which a plurality of optical signals having different wavelengths are transmitted on a single optical fiber transmission path has been making progress. Recently a WDM transmission system in which the number of multiplexes has increased to several hundred channels has been reported, while WDM transmission systems having 160 channels are already being manufactured at the business use level.
FIG. 30 shows an example of the structure of a conventional wavelength-division multiplexed optical signal transmitter provided with a multi-wavelength optical modulation circuit. The structure in this drawing is disclosed in, for example, a paper by N. Takachio et al. entitled “Wide area gigabit access network based on 12.5 GHz spaced 256 channel super-dense WDM technologies” in IEE Electronics Letters, Vol. 37, pp. 309–310, Mar. 1, 2001. In this drawing the wavelength-division multiplexed optical signal transmitter generates light in a plurality of wavelength channels by filtering (spectrum slicing) multi-wavelength light output from a multi-wavelength generation light source 981 using an optical demultiplexer 982. The light of each wavelength channel is then modulated respectively by a plurality of optical modulators 983-1 to 983-n, and the modulated signal light then undergoes wavelength-division multiplexing using an optical multiplexer 984 and is transmitted. Multi-wavelength light includes a plurality of different wavelength components and the wavelength components can be separated into each wavelength component with each of these being able to be used as an optical carrier for a different signal. Multi-wavelength light is obtained by modulating the phase of laser light using a single frequency. Note that it is also possible to obtain multi-wavelength light using a mode-locking scheme (see a paper by H. Sanjoh et al. entitled “Multiwavelength Light Source with Precise Frequency Spacing Using a Mode-Locked Semiconductor Laser and an Arrayed Waveguide Grating Filter”, IEEE Photonics technology letters, VOL. 9, No. 6, June, 1997). It is also possible to obtain super multi-wavelength light (super continuum light) by causing a non-linear effect in pulse light. A light source that generates multi-wavelength light efficiently from one or a plurality of seed laser diodes in the manners described above is known as a multi-wavelength generation light source. In a conventional wavelength-division multiplexed optical signal transmitter an arrayed waveguide grating (AWG) filter is used as the optical demultiplexer 982 and the optical multiplexer 984. However, an AWG has a cyclic transmission characteristic in that all wavelengths with the space of free spectral range (FSR) are transmitted. Therefore, when an AWG for channel demultiplexing that has the same channel spacing as the multi-wavelength light demultiplexes wideband multi-wavelength light having multiplicity in which, for example, the wavelength number is 1000 channels or more, as is shown in FIG. 31, the frequency band of the multi-wavelength light exceeds the FSR of the AWG resulting in a plurality of wavelengths being output from a single output port. Namely, in this type of AWG for channel demultiplexing it is not possible to demultiplex multi-wavelength light having a frequency band equal to or wider than the FSR into individual wavelength channels.
Moreover, examples of the multi-wavelength generation light source 981 include those that use amplified spontaneous emission light (ASE light) output from an optical fiber amplifier, and those that use repetitive short optical pulses.
When repetitive short optical pulses are used, as is shown in FIG. 32, the problem arises that a power level deviation is generated between each of the wavelength channels obtained by the spectrum slicing. If the power is not uniform for each wavelength, the cross-talk effect generated by the high powered wavelengths to the low powered wavelengths increases and excessive degradation may occur. Moreover, if the total power is decreased so that high powered wavelengths do not cause degradation due to the nonlinear effect, noise in the low powered wavelengths increases.